The present invention generally relates to programmable memory cell devices and more particularly to a so-called BIC memory cell device and a manufacturing method thereof.
A BIC (Breakdown-of-Insulator-for-Conduction) memory cell device is a programmable memory cell device comprising a MOS transistor having a drain region connected to a bit line via a thin insulator film and a source region connected to the ground (see for example, Sato N. et al. A New Programmable Cell Utilizing Insulator Breakdown. IEDM 85, pp.639-642, IEEE 1985). The gate of the MOS transistor is connected to a word line and the writing of a data is made by causing a breakdown in the thin insulator film by applying a high voltage to a selected bit line while at the same time applying a predetermined voltage to a selected word line. This high voltage is chosen such that the thin insulator film is destroyed with reliability at this voltage. Responsive to the breakdown of the thin insulator film, the drain of the MOS transistor is connected directly to the bit line. When reading out the data, another bit line voltage substantially lower than the first bit line voltage is applied to a selected bit line while applying the predetermined word line voltage to a selected word line. This second bit line voltage is chosen such that the breakdown of the thin insulator film does not occur even when such a voltage is applied to the bit line. When the addressed memory cell is the one already written with a data, the bit line is connected to the ground via the MOS transistor and the bit line voltage thus produced assumes a low level state. When the addressed memory cell is not written with the data, on the contrary, the bit line is insulated from the ground and thus the bit line voltage assumes a high level state.
Such a device has a simple structure and easily manufactured in a form of integrated circuit. Further, it can be manufactured by a step which is almost the same as the manufacturing step of a dynamic random access memory and thus the manufacturing cost can be reduced. Further, such a memory device is more reliable as compared to conventional programmable read only memories using fuse as the means for writing the data. Furthermore, the BIC memory cell device has a writing speed of less than 1 .mu.sec which is faster than any other programmable read only memories available at the present time.
In order to obtain a reliable operation in such a memory cell device, it is essential that the breakdown of the insulator film occurs uniformly at the high bit line voltage throughout a number of MOS transistors in the memory cell device. For this purpose, conventional BIC memory cell devices use a structure in which a penetrating hole, provided in an insulator layer covering the drain region of the MOS transistor so as to allow the connection of the bit line to the drain region via the thin insulator film, has a blunted cross section such that no sharp edge or kink is formed in the thin insulator film. When a sharp edge or kink is present in the thin insulator film, such a part tends to invite a concentration of electrical field and there is a substantial risk that the insulator film causes the breakdown prematurely at a low bit line voltage, for example when reading out the data from the memory cell. When such a premature breakdown occurs, erroneous writing of data is made.
In order to avoid such a problem and to secure a reliable operation, the conventional BIC memory cell device uses holes having the blunted cross section as already described. When manufacturing a BIC memory cell device having such a structure, holes for the source region and for the drain region are usually formed at the same time by etching. The sharp edge of the holes thus formed are then blunted by annealing. In such a procedure, however, there is a problem in that both of the holes, one for the source region and one for the drain region, are blunted by the annealing. As the blunted cross section is not necessary for the hole for the source region which is connected directly to the ground, and as such a blunted cross section occupies a substantial area in the substrate, there is a problem in the conventional BIC memory device that the memory cell device manufactured by such a process has an unnecessarily large size and is therefore disadvantageous for increasing an integration density when a memory cell array is constructed by assembling numerous BIC memory cell devices in a form of integrated circuit.